How Fast Does the Earth Rotate?

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# How Fast Does the Earth Rotate?

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The Earth is the third planet from the sun, and is a rocky planet with a solid body. The Earth rotates around its axis (an imaginary line which goes from the North Pole to the South Pole) in a counter-clockwise direction. The angle of the axis on which the Earth rotates is tilted from the angle at which the Earth orbits the sun: imagine the Earth standing straight up as it orbits the sun - but rotating on a slight lean. This is called the “axial tilt”.

Figure 1: Description of relations between Axial tilt, rotation axis, plane of orbit, and equator. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise. Image credit: Dennis Nilsson [CC-BY-3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

There are two ways in which scientists measure the amount of time that it takes a planet to rotate: the time it takes the Earth to rotate compared to the stars (this is called a sidereal day); and the time it takes the Earth to rotate compared to the Sun (this is called a solar day.)

The sidereal day of the Earth is 23.93 hours, while the solar day of the Earth is 24 hours. If you look at one point on the equator, and measure how much distance it is travelling over this period (40,075 km), you can calculate that Earth’s equator is travelling at a speed of 1,674.4 kilometres per hour, or 1,040.4 miles per hour as the Earth rotates.

Every planet in the solar system has its own rotation time. Earth’s solar day and sidereal day are nearly the same, but some other planets are very different. Mercury’s sidereal day is equivalent to 59 days, but its solar day is 176 days! Most planets have a solar day that is longer than their sidereal day, but not all of them. For example, Venus’s sidereal day is 243 days, but it’s solar day is 117 earth days! This is because Venus is one of the only two planets in the solar system (the other one being Uranus) that have what is called a “retrograde” rotation. This means that they rotate in the opposite direction to Earth and the other planets.

The rotation of the planets occurs because of how our solar system was formed. Our solar system began as a large cloud made up of gas and dust. The cloud slowly collapsed, and flattened into a giant disk that spun around extremely quickly (like a frisbee flying through the air!). The sun was at the centre, and the swirling gas and dust clumped together to form our planets. While the solar system was forming, lots of asteroids and comets were also spinning around in the cloud of gas and dust. They sometimes collided with the planets, and when they knocked into each other they caused each other to start spinning. This is what caused the planets to start rotating in the first place.

An interesting fact is that the Earth’s rotation is actually slowing down over time. This is caused by “tidal forces” between the Earth and the Moon. The side of the Earth that is closest to the Moon as it rotates is pulled more strongly by the tidal force of the Moon; the side of the Earth that is farther away from the Moon is pulled more weakly. The tidal forces pull on the water on the Earth, and this is what causes the tides to go in and out.

Scientists think that the Moon was formed when a large space object collided with the Earth. It knocked out a large chunk of rock that became the Moon. When the Moon first formed, it was much closer to the Earth. As the Moon’s gravity pulls on Earth, causing the tides, this creates friction that slows down how fast the Earth can rotate: this means that the Earth’s rotation is slowing down a little bit each year!

Useful Websites

NASA Space Place - Why is Earth rotating?

PhysLink.com -  Is the earth's rotation slowing down?

Windows to the Universe - The Earth’s Rotation